I suggest cycling the tank first :) makes it easier on your fish! that's 4-6 weeks running the tank, with either fish food flakes (a couple a day apparently) or some people use raw shrimp. you can ask people on here about that, if you don't already know how to! :)
You don't really want to add too much in the 5 gallon... most bettas make shrimp into nummy snacks, and any fish I could recommend with bettas should be in 10 gallons or more so not to stress him out :)
as for water changes, you could do a 20% weekly water change, with gravel siphoning. You'll definitely want a gravel siphon and a bucket! saves you work, and plus the betta won't have to be moved.